Re: [Linuxptp-users] Interpretation of ptp4l results

[Chandra M. <sma...@al...>]

Hi Richard,

Sorry for belaboring on this.

Can I then confirm the following mathematical definitions of the ptp4l repor, based on our mail threadt? This helps all of us (mostly rookies like me) to be on clear mathematical understanding of what is reported.

When ptp4l reports:  ptp4l[833.369]: rms    2 max    7 freq -156403 +/-   7 delay    28 +/-   0


Offset correction: the 'rms' (root mean squre - 2 ns in the above case) is the mean offset (calculated as squareroot of the mean of the offset squares) of all the offsets in the sample observed in the interval and 'max' (7ns) is the maximum offset observed within the sample.
Frequency correction: Mean frequency deviation observed is 156403 ppb with the standard deviation of 7 pbb (if so, is 7pbb at 3sigma?): likening it to gaussian/normal distribution
Path delay: the mean value is 28ns and the standard deviation is 0: likening it to the gaussian/normal distribution

Thanking you in anticipation,
Regards,
Chandra

(c) : +60.175508142
(O): +60.4.636.6412

"Knowledge speaks, Wisdom listens"



-----Original Message-----
From: Richard Cochran [mailto:ric...@gm...]
Sent: Monday, June 22, 2015 2:31 PM
To: Chandra Mallela
Cc: Miroslav Lichvar; lin...@li...
Subject: Re: [Linuxptp-users] Interpretation of ptp4l results

On Sun, Jun 21, 2015 at 11:09:08PM +0000, Chandra Mallela wrote:

> However, you mention that 'max' for the offset is simply the maximum
> offset observed in the sample. In that case, do frequency correction
> and offset correction values differ in their definitions (with 7ns in
> the offset defining the max offset in the sample whereas 7 pbb in the
> frequency showing the maximum deviation from the mean)?

These are two different parameters, and they are indeed defined differently.  The offset is the observed offset based on the result of the PTP.  The frequency is the output from the servo, and it is only indirectly related to the observed offset.  For example, using a weak proportional weight, you will have see a smaller range of frequencies.
Or if you prohibit resetting the clock, a large offset will cause a constant frequency value (steering using the maximum possible value).

HTH,
Richard

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